With a 10K resistor as R12, I got a VERY nice
volume that filled the entire room. The only bad
news is that when I remove the input signal, I get
about 400 hz motorboating.

Squire-

I believe R12 should be ten ohms. It provides a load at higher frequencies
where the speaker or headphone impedance might be too high for stability.

The motorboating is most likely feedback via the power supply line. Therefore,
the ten ohm series resistor should be higher (perhaps 100 ohms?), and the
filter capacitor at the IC's power terminal could be increased.

73, Fred, K4DII